1. Field of the Invention
The present invention relates to a multiple element printer which prints by moving a printhead having a plurality of printing elements, a receiving medium in a main scan direction and moving in a sub scan direction, and a method of adjusting a multiple element printer.
Further, the present invention relates to a multiple element printer realizing high print density by controlling an image printing operation of a plurality of printheads which each include a plurality of printing elements.
2. Discussion of Background
According to a print device which operates by relatively moving a printhead and a receiving medium, feed accuracy of the receiving medium and positional accuracy of a plurality of printing elements arranged at the printhead are significant factors affecting quality.
According to such a printhead, although excellent image quality can be obtained by densely arranging the printing elements, the high density formation of the printing elements is actually limited. Hence, the high density formation is realized by arranging a plurality of printing elements at intervals of a printing resolution multiplied by an integer in a direction of feeding a receiving medium (referred to as sub scan direction) and scanning multiple times in a direction of moving the printhead (referred to as main scan direction) and feeding the receiving medium in the sub scan direction by a unit of a minimum resolution. FIG. 19 is an explanatory view illustrating the arrangement of printing elements and the printing operation of a conventional printhead with an example of an interval between the printing elements being k=4 (dots) and a number of the printing elements being n=5. The receiving medium is fed by 1 dot in the sub scan direction at every scanning of the printhead in the main scan direction. In the case of this example, printed dots are formed at an interval of a unit of a minimum resolution, that is, at an interval of 1 dot by 4 times of the main scanning (t=4). When the 4-th main scanning (t=4) has been finished, the receiving medium is fed by {(n-1)k+1} dots in the sub scan direction (t=5) and feeding operation of the printhead in the sub scan direction by 1 dot, that is, the operation of the t=1 through t=4 is repeated thereby achieving high density printing.
However, there actually is a very small dispersion in print positions of the respective printing elements in the sub scan direction. The dispersion is caused by a deviation in arrangement of the printing elements per se or is caused by a deviation in forming dots on the receiving medium. The deviation in the print position is significantly manifested at portions of continuous print regions by one printing element coupled with portions of continuous print regions by other printing element whereby the image quality may be deteriorated. Further, in respect of the amount of feed in the sub scan direction, the amount of feed by 1 dot and the amount of feed by {(n-1)k+1} dots are mixed. The feed accuracy of the receiving medium is dispersed depending on the amount of feed in the sub scan direction and therefore, when the amounts of feed in the sub scan direction of two kinds or more are mixed, the difference in the accuracy may deteriorate the image quality.
Hence, there has been proposed an interlace printing method to reduce such a dispersion of printing elements and the difference in the feed accuracy. An interlace printing method is described in, for example, U.S. Pat. No. 4,198,642. The interlace printing method is a method where a relationship between an interval k of printing elements and a number thereof is specified and a receiving medium is fed in the sub scan direction by a constant pitch. As conditions of this case, k and n are mutually prime with each other and the feed pitch is set to n by which the interlace printing can be realized. FIG. 20 is an explanatory view for explaining the interlace printing method where the interval between printing elements is set as k=4, the number thereof is set as n=5 and the feed pitch of paper in the sub scan direction is set as n=5. In FIG. 20, notation L designates a print line and the effective interlace printing is conducted at a 4-th main scanning (t=4), that is, a 13-th print line and thereafter.
By adopting the above-described printing method, the printing elements printing contiguous lines are made to differ from each other whereby a disturbance in image quality derived from the positional dispersion of the printing elements is diminished and the deviation in the feed accuracy of a total of a print region is made uniform. However, it is quite difficult to ensure the feed accuracy of the receiving medium and even if the interlace printing is carried out, the image quality may be deteriorated depending on the moving accuracy. FIG. 21 is an explanatory view for explaining problems in the conventional interlace printing method. FIG. 21 shows a case where the interlace printing is carried out using a printhead conducting the operation illustrated by FIG. 20 and is an example where a resolution of a drive source such as a motor or the like is poor; and therefore, the feed accuracy of the receiving medium is low. When an error of (-.alpha.) is caused with respect to n dots of the feed amount of a receiving medium, the amount of feed at one time is shortened. Although firstly interlaced print lines of 8 lines are completed at a 4-th main scan (t=4), a gap is actually caused between a 16-th line and a 17-th line which constitutes a streak and deteriorates the image quality. Thereafter, the streaks emerge at every 5 lines. Although the error of the feed amount in the negative direction has been described in the above example, the streaks are also manifested in the case where the error is caused in the positive direction.
The same phenomenon is caused also with respect to the accuracy of attaching the printhead. When the printhead is attached with a certain inclination with respect to the sub scan direction, the actual interval between printing elements in the sub scan direction is contracted in respect of the feed amount of receiving medium. Accordingly, as a result, the phenomenon similar to that where the error of the positive direction is caused in the feed accuracy of the receiving medium, which also appears in images as streaks.
The conventional multiple element printers are constructed as described above and therefore, even if the interlace printing is carried out, when the resolution of the drive source such as a motor or the like is poor and therefore, the feed accuracy of the receiving medium is low, or the attachment accuracy of the printhead is poor, streaks occur in printed images to diminish image quality.
According to the above-described conventional multiple element printers, the high density printing system or the interlace printing system is conducted by adopting the plural time scanning and the feed by a unit of a minimum resolution. Therefore, a very fine deviation in the print position in the sub scan direction of the printing element (that is caused by a dispersion in the characteristic of the printing elements, a deviation in arrangement of the printing elements, a deviation in printing dots on the receiving medium, or the like) or a deviation in feed accuracy of the receiving medium (that is caused by the mixed feed amounts of two kinds or more in the sub scan direction) which is significantly manifested at coupled portions of continuous print regions of the printing elements, deteriorates the image quality. Further, even with the interlace printing system as a measure for such drawbacks, an increase in the number of the printing elements determining the printing speed, which is derived from the high density of the printing elements or an increase in the length of the printhead, increases the dispersion of the characteristic of the printing elements, deteriorates the yield of the printhead per se and increases the cost of the device.